* Added Mercurial support.
* Added a CLI.
* Renamed `Version.from_git_describe` to `Version.from_git`.
* Changed behavior of `Version.serialize` argument `with_metadata` so that,
by default, metadata is excluded when post and dev are not set.
* Added `with_dirty` argument to `Version.serialize` and removed `flag_dirty`
argument from `Version.from_git`. The information should always be collected,
and it is up to the serialization step to decide what to do with it.
* Added `Version.from_any_vcs`.
* Removed `source` attribute of `Version` since some VCSes may require multiple
commands in conjunction and therefore not have a single source string.